One of the things Jerry Gray did in this recent interview (video link) is shed some light on what the Titans wanted in a safety, namely that they were looking for somebody to play in the Ruby package. This rung a distant bell in my head, and a quick scan of my bookmarks showed the answer: Matt Bowen wrote about how Gregg Williams, whom Gray served as defensive coordinator in Buffalo, used the Ruby package, a 3-2-6 alignment, to create exotic blitzes. See this post and this one for examples of what Williams does.
I'm not sure exactly what role Gray is looking for the safety to fill, but I'm guesing the dime back. Anthony Smith played that at times last year, as did Chris Hope once Babineaux took over at strong safety, but I'm guessing Gray wants a player with some specific traits for this role and others neither of those players precisely has. The Titans lined up in the Ruby package on defense something like 90 times last year, or roughly 8% of the time, six plays a game. That's not a whole lot compared to the twelve starting positions, but it is enough for Gray to be looking for the right player to fill that role.